In general, an image processing device that performs image processing for displaying an object on a display is provided with a special processor (hereinafter, referred to as “graphic accelerator”) in addition to a main processor that performs control on the entire image processing device and the like. When this graphic accelerator performs image processing, such as drawing processing of an object, image processing can be realized at a higher speed than when the main processor performs the image processing.
The image processing by the graphic accelerator includes processing called clipping for displaying only a part of an object included in a designated part of a display area on the display (e.g. see Patent Document 1) The following is a description of a flow of general image processing including this clipping performed by the graphic accelerator, with the reference to FIG. 9.
As shown in FIG. 9, the graphic accelerator reads, from a memory, coordinate values of each vertex of an object expressed in a local coordinate system (Step S901). The graphic accelerator performs coordinate transformation that converts each of the read coordinate values to be expressed in a world coordinate system (Step S902). The graphic accelerator reads, from memory, data piece of each vertex, such as a color (R, G, B), a transmittance (α), a texture coordinate (U, V) (hereinafter, these pieces of data are collectively referred to as “attribute data”), and performs illumination calculation based on the read attribute data piece of each vertex and each coordinate value expressed in the world coordinate system (Step S903). Subsequently, the graphic accelerator performs viewport transformation which is coordinate transformation that converts from the world coordinate system into a viewing coordinate system (Step S904), and performs perspective transformation which is coordinate transformation that converts from the viewing coordinate system into a display coordinate system (Step S905). Lastly, the graphic accelerator performs clipping to delete part of data protruding from the display area (S906).    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2001-209369